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Abstract

A reinforcing splint between electromagnetic shields on a printed wiring board can significantly reduce the bending strain in the board area within the shields, especially at locations near the shield boundaries linked by the splint. This strain reduction improves the reliability of handheld products subjected to keypress and drop.

Copyright

Motorola Inc 03/08/01

Country

United States

Language

English (United States)

This text was extracted from a RTF document.

This is the abbreviated version, containing approximately 100% of the total text.

REINFORCING SPLINT BETWEEN SHIELDS ON PRINTED WIRING BOARD

by David Read, Krishna Jonnalagadda, Andrew Skipor and William Olson

SUMMARY

A reinforcing splint between electromagnetic shields on a printed wiring board can significantly reduce the bending strain in the board area within the shields, especially at locations near the shield boundaries linked by the splint. This strain reduction improves the reliability of handheld products subjected to keypress and drop.

PROBLEM

Handheld products occasionally fail electrically in the field because of mechanical stresses induced by drop or keypress. The most common mechanism for such failures is broken solder joints between integrated circuits (IC), often in the form of ball grid arrays (BGA), and their supporting printed wiring board (PWB). The solder joints break because they are stretched to failure when the PWB bends excessively.

Designs of handheld products always attempt to support and protect the PWB. Strategies have included screws holding the PWB to the product case pads and spacers between multiple PWB and between PWB and housing; and "bed frame" spacers between multiple PWB or between PWB and housing. Reliability testing has shown that additional methods can be applied to further improve stiffness mechanical reliability.

SOLUTION

We propose the addition of reinforcing splints between neighboring shields around critical failure-prone components on PWB, (Figure 1 ). The splints suppress strains within the shields near the edges, thus reducing the incidence of broken solder joints. A splint could be made of thin metal, similar to that used to construct electrical shields. It could be flat, if the neighboring shields have the same height, or it could be shaped to fit the height differences. It could include folds or bends for additional rigidity. It could be soldered or glued in place.

Use of the splint captures mechanical reinforcement from existing electrical shields surrounding critical components. It suppresses strains produced inside the shields by the elastic hinge that develops between two neighboring shields. It can be added without changing the PWB design. Figure 3 shows an experimental test of the splint.

VERIFICATION

In both static and dynamic tests, the splint significantly reduced the strain in the PWB, as measured by strain gages. Furthermore, even though the splint is a local reinforcement, it has a noticeable effect on the behavior of the board as a whole. In same cases, reductions of about 50 percent in the average strain measured in drop.